Role of stratification of immunohistochemical markers in primary lung carcinomas: Experience in a south indian tertiary care center

Background: Accurate subtyping of lung carcinomas is essential owing to therapeutic implications. Aim and Objectives: To assess the role of immunohistochemical markers in the classification of primary lung carcinomas in resource-poor setting. Material and Methods: This was a 6-month retrospective study from January 2019 to June 2019 carried out in a tertiary care cancer centre. Biopsy confirmed cases of lung carcinoma were retrieved. Immunohistochemistry (IHC), wherever performed, was reviewed and the cases were typed. IHC markers were used in a tiered manner comprising TTF-1, Napsin A, p40, p63, Synaptophysin and Chromogranin A. CD56, Cytokeratin 7 (CK7), Pancytokeratin (CK) were performed in selective cases. Results: Two hundred and sixty-four (264) cases of primary lung carcinomas comprising of seven lobectomies and 257 small biopsies were studied. Prior to IHC, the distribution of cases was as follows: Small Cell Carcinoma (SCLC) (6.8%, n=18), non-small Cell Lung Carcinoma (non-SCLC) (89.4%, n=236) further divided into Adenocarcinoma (ADC) (35.9%, n=95), Squamous Cell Carcinoma (SCC) (19.8%, n=52) and non-SCLC, unclassifiable (33.7%, n=89). Non-SCLC, unclassifiable after use of IHC was non-SCLC favoring ADC (20%, n=53), non-SCLC favoring SCC (7.9%, n=21). One case of non-SCLC (adenocarcinoma) with SCLC (0.4%) on biopsy; one case of non-SCLC on biopsy showed both squamous and adenocarcinoma components (0.3%, n=1). Two cases were of adenosquamous (1.1%, n=2) and three cases of large cell neuroendocrine carcinomas (n=3,1.1%). Salivary gland type tumors comprised 6 (2.2%) cases. Eight cases (3%) could not be reclassified despite adequate IHC, and hence were retained in non-SCLC, NOS category. Conclusion: IHC panel of TTF-1, Napsin A, p40, p63, Synaptophysin, Chromogranin A, CK, CK7 used judiciously and in tiered manner aids in accurately subtyping most primary lung carcinomas.